Filters are commonly used in existing pipeline systems. The filter elements in the filters will become clogged by debris carried in the medium and need to be cleaned from time to time by closing the hand-operated isolating valves to shut off that stretch of piping and taking out the filter element to flush off the debris. This process is time-and labor-consuming. The German Patent DE 3030447 discloses a three-way valve with a floating spheroidal valve core, with a filter element incorporated in its valve core. This valve can have its filter element reverse-flushed on line. However, it has the following two major shortcomings: (1) Owing to the floating spheroidal valve core, the valve can only have two sealing seats disposed in two ports of its valve body along a rectilinear flow path, it can therefore have only two modes of flow, i.e. filtered flow and reverse-flushing. It is not intended, therefore it is not possible, to shut off the medium source. Hence a hand-operated isolating valve has to be installed in front of this valve. (2) The filter element is retained in position by two washers, two seal-rings and a retaining ring, therefore the flow cross-section is restricted; the assembly comprises a multiplicity of parts and thus a relatively high cost is incurred.
In the usual practice of pipeline arrangements, it normally requires a number of two-way valves. In FIG. 1b the square symbol T represents a tank, having two hand-operated valves V.sub.1 and V.sub.2 located in its up-stream and down-stream pipeline for controlling the in-flow and out-flow of the medium. There is also a hand-operated by-pass valve V.sub.3. FIGS. 1c and 1d show respectively systems with two tanks T.sub.1 and T.sub.2 and three tanks T.sub.1, T.sub.2 and T.sub.3, each with three valves V.sub.1, V.sub.2 and V.sub.3 connected in the pipeline. In FIG. 1a the system contains an automatic control valve 11 and three hand-operated valves 13, 14 and 15. Valve 13 stops the in-flow of the medium to the automatic control valve, valve 14 is a by-pass valve, and valve 15 controls the out-flow of the medium from the automatic control valve, either for the medium retained in the valve or in the pipeline arrangements in the rear of the valve. Obviously these systems are complicated. It could be much simplified if the illustrated three valves are replaced by a single valve. This is not possible by using the prior art valve.